Is it possible to get full screen mode with two apps in OS X Yosemite?
Even with a 3rd party tool perhaps?
The behavior I want is to "lock" two apps (which I use HyperDock to lock to one half of the screen) to a single Desktop/Space, so when I start another app (say, via Spotlight), it doesn't stack on top of the two apps that I have open in this Space.
Any app set to full screen will automatically go to a new Space of its own, to the right of any existing spaces - see this example with one Safari Window & Mail set as fullscreen, to the right of my 6 existing numbered Spaces…
You cannot assign any hotkey to go directly to a fullscreen app, only to a numbered Space, but default is Ctrl ⌃ [Num] & Ctrl ⌃ ← or → to go to last/next, so to get there quickly I use Ctrl ⌃ 6 / →
You can set those up in System Prefs > Keyboard > Shortcuts > Mission Control.
Other than in fullscreen, you can set any app to a specific Space by launching it in that Space or dragging it there by the header bar + key combo, then right click in the Dock & Options > Assign to… This Desktop.
Any app which doesn't have an assigned Space will always open in the current one, unless the current Space is an auto-generated fullscreen Space.
If I'm understanding it correctly, you want to have two windows share one Desktop/Space fullscreen, and when another app is open, it'll go to another Desktop/Space? If that's the case, then this behavior actually comes default with OS X.
To get this behavior:
Now that Desktop/Space has two windows in fullscreen mode, and when you're on that Desktop/Space, any new app opened will go to another Desktop/Space.
I use Apptivate to bind custom commands to open apps.
For example, I have this combinations among others:
That way I don't have to worry what app was last used and just insert the shortcut to open the app that I want.